This invention relates to the field of metal electroplating and, more particularly, to the utilization of particular surfactants and surfactant compositions in the preparation of electroplating baths and use of such baths for electroplating metals onto suitable surfaces.
The art of the electroplating or electrodepositing of a metal or metal alloy from solution onto surfaces of a suitable substrate or workpiece is, of course, well known and well developed for a wide range of metals, alloys and substrates. The fundamental process involves immersion of the workpiece in a suitably prepared electroplating bath and application of a voltage across the cathodic workpiece and an anode which also is immersed in the bath. The essential bath components comprise an aqueous admixture of a source of the metal ion to be deposited and an electrolyte. Many other optional bath components are, of course, known and used to attain particular effects in electroplating such as increased efficiency of plating, stabilization of bath components, pH adjustment, improved brightness or ductility or adherence of the metal deposit, and the like.
Ideally, all components of an electroplating bath will be water soluble at the concentrations and conditions employed so that an equal distribution of components can be attained. As is known, however, this ideal is not usually realized and many of the special components employed to attain particular effects are either insoluble in water or only difficultly soluble. For this reason, it is common to include as an essential component in the electroplating bath a surfactant (surface active agent) to aid in solubilizing bath components and/or to promote stable, even distribution of bath components.
The operating conditions for electroplating from aqueous solution can vary widely depending upon the metal to be plated, substrate, bath composition, and the like. In general, the operating temperature of the bath is desirably high so as to increase plating efficiency. However, the ability to employ high temperatures in electroplating is severely mitigated by limitations imposed by the compositional make-up of the electroplating bath. In particular, surfactant-containing aqueous electroplating baths have associated with them a particular "cloud point", i.e., a temperature at and above which the surfactant component oils or salts out from solution (exhibited by turbidity or cloudiness in the bath). Attempted operation at or above the cloud point of the bath will lead to extremely poor electroplated deposits because of the non-uniformity of bath component composition.
In formulating aqueous electroplating baths, additives and techniques employed to attain a particular advantage can have adverse effects on other bath or operating properties. A good example of this is seen in acid zinc electroplating baths. Early formulations of such baths utilized an ammonium salt as the soluble electrolyte and a non-ionic surfactant, and as so formulated, were capable of operation at relatively high and efficient temperature. Later formulations of these baths were designed to eliminate or substantially reduce ammonium ion (and the disposal problems associated with its ability to complex heavy metals) by using a non-ammoniated electrolyte and other compounds (e.g., boric acid) to compensate for lost ammonium ion buffering capacity. These reformulations, however, resulted in substantial lowering of the cloud point of the bath such that attempted operation at temperatures used in ammoniated formulations caused precipitation of the non-ionic surfactant and consequently ineffective plating. The need to operate these baths at reduced temperatures to avoid these problems is a limitation on plating efficiency and on operating flexibility.
For any fundamental bath formulation, it is highly desirable to have flexibility in operating temperature conditions. In this way, other operating conditions such as current density and current efficiency and various formulation characteristics can be optimized without concern for inherently limiting restrictions imposed by operating temperatures and cloud points. Moreover, bath operating temperature flexibility provides insurance against unexpected temperature rises (e.g., climate changes or equipment malfunction) which, if exceeding the cloud point, would otherwise lead to poor plating and/or require a complete shut-down of the operation. In accomplishing this flexibility through choice of particular surfactants which exhibit a desirably high cloud point in the desired media, it is of course necessary that the surfactant be capable of serving its intended purpose and not otherwise adversely affect bath, plating or deposition characteristics.